The present invention relates to the use of abrasives for lapping gun barrels, and in particular relates to a process for embedding abrasives in the surface of bullets for lapping of gun barrels.
All shooters, whether they are police officers, olympic shooters, or weekend enthusiasts, have one common goal: hitting their target accurately and consistently. Accuracy and consistency in shooting depend largely on the skill of the shooter and the construction of the firearm and ammunition.
The accuracy of a firearm can be enhanced by careful attention to the component parts of the firearm. Thus, accuracy can be enhanced by improving the bedding of the barrel and action; securely mounting sight and scope bases; trigger adjustment; checking for eccentricities of and indexing cartridge casings; case neck turning and tension of bullet release; use of match grade bullets; careful cartridge preparation; and providing a barrel capable of providing a smooth, even passage for a bullet.
It is well known that gun barrels are manufactured with tolerances which may vary depending on the use of the firearm. Factory/military gun barrels typically include manufacturing machine marks in the bore, poor surface finish, small burrs and sharp edges from the rifling process, non-uniform rifling twist rate, or uneven bore diameters. Factory type barrels can, as a result of the manufacturing process, be tapered, with the breech end smaller than the muzzle end or vice versa. Further, it is not unusual for gun barrels to have "tight" spots at varying locations along the length of the bore. These "tight" spots and taper variations represent dimensional deviations of the bore which can be within tolerances for the barrel, yet still adversely affect accuracy since they influence the travel of the bullet in the barrel and provide a locus for the deposit and build up of metal from the bullets fired (referred to in the art as "metal fouling"). The greater the deviation and fouling, the greater the loss of accuracy. Such loss of accuracy can produce wide variations in bullet trajectory, which in target shooting produces a large pattern or spread of bullet holes on the target (and a lower score than would be expected if the spread of bullet holes were small, producing a "tight" group). These variations in the dimensions, finish, and twist of the bore should therefore be considered, and corrected, if possible, in order to obtain optimum accuracy.
Conventional lapping has been used as a finishing process for custom barrels to remove some or all of the machining marks, burrs, sharp edges, and dimensional deviations in the bore. Conventional lapping consists of placing an abrasive/lapping compound on a bore-cast lead slug mounted on a rod and repeatedly passing the slug through the bore of the newly cut barrel to remove the unwanted metal and polish what is left. The process takes place in steps, beginning with a coarse abrasive and lapping compound, and proceeding through abrasives of increasingly finer grit. However, this method is disadvantageous because it does not duplicate the environment of pressure and bearing surface which takes place during the passage of a bullet through the barrel. Further, conventional hand lapping, while capable of producing excellent results, tends to be a very time consuming and expensive process.
The problems of conventional lapping have been solved to a certain extent by a process known as "pressure (fire) lapping." In pressure (fire) lapping, the bore of a fully assembled gun is cleaned and all deposits of copper or lead are removed, using cleaning compounds intended for that purpose. The barrel is then checked to determine the location of tight or rough spots by pushing a lubricated soft slug through the bore from end to end. A series cf abrasives are used, as in conventional lapping, however in pressure (fire) lapping bullets are impregnated with the desired abrasive by rolling individuai lubricated bullets with a desired abrasive between two steel plates under pressure. After embedding abrasive in each series of lap bullets, the abrasive must be cleaned off of the plates before beginning to embed the next series of lap bullets.
Excess abrasive is wiped off the impregnated bullets, which are then loaded into cartridges with a minimal powder charge and a plastic wad positioned against the base of the abrasive-impregnated bullet. The cartridges with the abrasive-impregnated bullets are fired through the barrel on a series, with the coarse abrasive impregnated bullets being fired first, followed may a series of each of the finer grits. In between each type of abrasive or every ten rounds or so, the gun is cleaned and checked to determine the extent to which the dimensional irregularities are being removed. Anywhere from about 10 to about 40 bullets impregnated with each abrasive may be required to substantially reduce or completely remove the observed tight spots.
Unlike conventional lapping, pressure (fire) lapping duplicates the environment which exists in the bore during the firing of a bullet. Conventional lapping does not apply torque to the bearing surfaces; however, because the identical conditions of heat and pressure are recreated and because torque is applied by the lapping bullet which bears against the rifling during pressure (fire) lapping, the irregularities in the rate of twist of the rifling, which are typically found at the muzzle and breech, will approach uniformity and bearing edge of the rifling land is smoothed. Additionally, in pressure (fire) lapping abrasive marks are properly aligned with the direction of bullet travel, and reamer and tool marks in the bore are reduced and smoothed. Many of those who have used pressure (fire) lapping report noticeable to significant improvements in accuracy. However, pressure (fire)lapping is also a tedious process, and typically takes approximately 8 hours per gun barrel. In particular the method of impregnating bullets by individually rolling in abrasive and lapping compound between two plates under pressure is particularly tedious and time consuming, especially when one considers that 100 bullets, or more, may need to be impregnated. Furthermore, the curvature of some rifle bullets will require a 2-step process, whereby the base section and the nose section are impregnated with abrasive separately.
Accordingly, the need exists for an improvement to the pressure (fire) lapping process which will reduce the time required to fire lap a gun barrel. In particular, a need exists for an easier and more efficient method of impregnating bullets with abrasive for use in pressure (fire) lapping.